The present disclosure relates to technology of automatically controlling (automatic power control: APC), in a light source which emits a light beam for forming an electrostatic latent image, the light quantity of the light beam emitted from the light source.
Formation of an image based on an electrophotographic system includes a step of forming, on a photosensitive drum, an electrostatic latent image of an image indicated by image data, a step of supplying a toner to the electrostatic latent image and forming a toner image, a step of transferring the toner image to a sheet, and a step of fixing, to the sheet, the toner image that was transferred to the sheet.
In the step of forming the electrostatic latent image, the light beam emitted from the light source by controlling the emission of the light source is deflected with a polygon mirror, and a main scanning line is drawn on the rotating photosensitive drum. As a result of repeating the foregoing processes, an electrostatic latent image is formed on the photosensitive drum.
When the magnitude of the drive current of the light source is fixed upon drawing the main scanning line on the photosensitive drum, the light quantity (or intensity) of the light beam irradiated on the photosensitive drum will differ according to the irradiation position on the photosensitive drum. The reasons for this include the distance between the photosensitive drum and the polygon mirror being different at the center part and the end part of the photosensitive drum (the distance between the polygon mirror and the center part of the photosensitive drum is shorter than the distance between the polygon mirror and the end part of the photosensitive drum), and the optical property of the condenser lens disposed between the polygon mirror and the photosensitive drum.
When the light quantity of the light beam irradiated on the photosensitive drum differs according to the irradiation position on the photosensitive drum, there will be unevenness in the density of the image.
Thus, the magnitude of the drive current of the light source during the main scanning is adjusted so that the light quantity of the light beam irradiated on the photosensitive drum becomes constant on the photosensitive drum. For example, proposed is technology of generating a pulse width modulation (PWM) signal which indicates the light quantity of the light beam to be emitted from the light source, smoothing the PWM signal and generating an analog signal of a magnitude corresponding to the light quantity of the light beam to be emitted from the light source, and adjusting the magnitude of the drive current of the light source based on the analog signal.
The light quantity of the light beam to be emitted from the light source is automatically controlled (APC) based on the signal output by the light-receiving unit that received the light beam. This is because a laser diode that is used as the light source is a semiconductor laser, and the light quantity of the light beam emitted from the light source will differ depending on the temperature, even if the drive current is the same.
When the analog signal is generated by smoothing the PWM signal, ripples are generated in the analog signal in correspondence with the cycle of the PWM signal. As a result of the light quantity of the light beam emitted from the light source fluctuating due to the ripples during the period that the APC is being executed, the APC cannot be executed accurately, and, consequently, there is a possibility that the density of the image may become uneven.
When the circuit for smoothing the PWM signal is a low-pass filter configured from a CR filter, it is possible to prevent the generation of ripples in the analog signal if the time constant of the CR filter is increased. Nevertheless, if the time constant is increased, the response time from the time that the PWM signal is input to the CR filter and the analog signal is emitted will increase, and this will lead to the delay in the control of the light quantity of the light beam. Accordingly, there is a limit in increasing the time constant of the CR filter.
An object of the present disclosure is to provide an image forming apparatus capable of preventing the generation of ripples in the analog signal which is generated by smoothing the pulse signal and serves as a base of the drive current of the light source that emits the light beam upon executing the APC of the light quantity of the light beam for forming an electrostatic latent image.